Transantarctic Mountains, Antarctica

Palaeogeography, Palaeoclimatology, Palaeoecology 213 (2004) 65–82 www.elsevier.com/locate/palaeo Neogene vegetation of the Meyer Desert Formation (Sirius Group) Transantarctic Mountains, Antarctica Allan C. Ashwortha,*, David J. Cantrillb aDepartment of Geosciences, North Dakota State University, Fargo, ND 58105-5517, USA bDepartment of Palaeobotany, Swedish Museum of Natural History, Box 50007, SE-104 05, Stockholm, Sweden Received 5 November 2003; received in revised form 28 May 2004; accepted 2 July 2004 Abstract A tundra vegetation consisting of at least 18 plant species is described from the Meyer Desert Formation which outcrops along the Beardmore Glacier in the Transantarctic Mountains, about 500 km from the South Pole. The fossils include pollen, seeds, fruits, flowers, leaves, wood, and in situ plants, of which wood and leaves of Nothofagus and a pollen assemblage had been previously reported. The plants include a cryptogamic flora of mosses and liverworts, conifers, and angiosperms in the families Gramineae, Cyperaceae, Nothofagaceae, Ranunculaceae, Hippuridaceae, ?Caryophyllaceae, and ?Chenopodiaceae or ?Myrtaceae. The plants grew in a weakly-developed soil formed within a complex periglacial environment that included moraines, glacial outwash streams, well-drained gravel ridges, and poorly drained depressions in which peat and marl were being deposited. D 2004 Elsevier B.V. All rights reserved. Keywords: Antarctica; Neogene; Nothofagus; Palaeoecology; Biodiversity; Palaeoclimate 1. Introduction yophyllaceae). Both species occur in the area from the islands of the Scotia Ridge, along the west coast of the The Antarctic flora today is dominated by crypto- Antarctic Peninsula south to the current southernmost gams, most of which only grow in the most sheltered site on Alamode Island, Terra Firma Islands at coastal locations north of 658S. Only two species of 68842VS. On the eastern side of the Antarctic vascular plants are known, a grass Deschampsia Peninsula, adjoining the frozen Weddell Sea, vascular antarctica Desv. (Poaceae) and a herbaceous caro- plants extend only as far south as 658 (Kappen and phyllid, Colobanthus quitensis (Kunth.) Bartl (Car- Schroeter, 2002). Both D. antarctica and C. quitensis are considered to have colonized the Antarctic Peninsula during the Holocene. Convey (1996) felt * Corresponding author. Fax: +1 701 231 7149. that the main reason why only two species had E-mail address: [email protected] successfully invaded was because of the geographic (A.C. Ashworth). isolation of Antarctica. The asymmetry in the geo- 0031-0182/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2004.07.002 66 A.C. Ashworth, D.J. Cantrill / Palaeogeography, Palaeoclimatology, Palaeoecology 213 (2004) 65–82 graphic ranges of the species on the Antarctic each of a freshwater bivalve and a lymnaeid gastropod Peninsula, however, suggests that climatic factors (Ashworth and Preece, 2003). The new fossil plant play an important role in their distribution. Support materials reported here include moss stems and for the importance of climate in limiting distribution leaves, in situ cushion plants, and fruits, stems and comes from the failure of experiments to transplant flowers of several species of vascular plants. The the species further south (Lewis Smith, 1994). discoveries indicate a more diverse Neogene flora The simple vegetational communities and the low than was previously known and, together with the plant diversity seen today contrast with the rich fossil previously discovered fossils, enable a more complete record. Diverse forests of conifers (podocarps and interpretation of the Neogene vegetation and land- araucarians) and angiosperms (Cunoniaceae, Notho- scape in the Transantarctic Mountains. fagaceae, Proteaceae, Lauraceae) inhabited the con- tinent during the Late Cretaceous and Paleogene Periods (Dettmann and Thomson, 1987; Askin, 2. Stratigraphy and age 1992; Cantrill and Poole, 2002). The geographic isolation associated with the breakup of Gondwana, The Meyer Desert Formation (MDF) is the name and the development of the Antarctic ice sheets, given to the mostly nonmarine glacigenic deposits resulted in radical changes to the vegetation. How- forming the upper part of the Sirius Group, formerly ever, the nature and the timing of extinctions caused the Sirius Formation, in the Meyer Desert and by climatic cooling is still being debated largely due Dominion Range region of the Transantarctic Moun- to the paucity of Neogene fossil sites (Cantrill, 2001). tains (Mercer, 1972; McKelvey et al., 1991; Webb et Mercer (1972, p. 432) was the first to suggest that al., 1996). The deposits are gently inclined to the an examination of the contact between the Sirius north and are fractured by numerous normal faults Formation, a formation he named, and the underlying with throws of up to 300 m (McKelvey et al., 1991). bedrock in the Oliver Bluffs, Dominion Range, might The MDF occurs in discrete, terraced sedimentary result in the discovery of d...a palaeosol, or traces of packages, with a combined thickness of about 185 m. vegetation.T His intuition proved to be correct. Each of the packages is unconformable on the Nothofagus wood was described by Carlquist (1987) bedrock of a multi-tiered, glacially-scoured erosion and Webb et al. (1987) and pollen by Askin and surface, representing several distinct episodes of uplift Markgraf (1986). Further descriptions of the wood, and erosion followed by deposition. pollen, and well-preserved leaves of a new species of The fossils are from beds within the topographically Nothofagus, Nothofagus beardmorensis, were made lowest and stratigraphically younger part of the by Hill and Truswell (1993), Webb and Harwood formation exposed in the Oliver Bluffs, on the eastern (1993), Hill et al. (1996), and Francis and Hill (1996). flank of the upper valley of the Beardmore Glacier at Today, the climate of the Oliver Bluffs is too cold and latitude 85807VS, longitude 166835VE. The Oliver dry to support vascular plant or cryptogram growth. Bluffs, at an elevation of 1760 m above sea level, are The reports of Neogene fossil wood, leaves, and located about 170 km south of the confluence of the pollen of Nothofagus from a site so far south are one Beardmore Glacier with the Ross Ice Shelf and about of the most important palaeobotanical discoveries to 500 km north of the South Pole (Fig. 1). The new fossils have been made in Antarctica. The discoveries are from a horizon about 20 m above the base of an provided the basis for palaeoclimatic interpretations erosional spur located near the northern end of the that became significant in an ongoing debate about the bluffs (Fig. 2A). The section is about 900 m north of the relative stability of the Antarctic ice sheets. one from which leaves of Nothofagus beardmorensis An expedition to the Oliver Bluffs during the 1995 were described (Hill et al., 1996). The section consists austral summer resulted in the discovery of new fossil of 85.5 m of interbedded diamictites, coarse cross- materials. These included two species of curculionid bedded sandstones, and wood-bearing siltstones and beetles (Ashworth et al., 1997; Ashworth and sandstones. On the scree slope immediately north of Kuschel, 2003), one species of a cyclorraphid fly the spur, we discovered a thin lithified peat interbedded (Ashworth and Thompson, 2003), and one species between thin marlstone deposits (Fig. 2F), at a similar A.C. Ashworth, D.J. Cantrill / Palaeogeography, Palaeoclimatology, Palaeoecology 213 (2004) 65–82 67 Fig. 1. Locality map of the fossil site at the Oliver Bluffs on the Beardmore Glacier in the Meyer Desert region of the Dominion Range, Transantarctic Mountains. stratigraphic level to the wood-bearing siltstones and Group in the Beardmore region, outcrops about 90 km sandstones. The horizon is also at about the same level down-glacier from the Oliver Bluffs. Webb et al. (1996) as a palaeosol identified as the lower of two Viento interpreted the non-stratified and stratified drift with types by Retallack et al. (2001, Fig. 5). McKelvey et al. dropstones as glaciomarine sediments deposited by (1991) interpreted the diamictites as lodgement tills grounded and floating ice in a wide fjord. The deposited by the ancestral Beardmore Glacier during a Cloudmaker Formation is overlain by the MDF series of advances and retreats; they interpreted the implying that the fjord infilled with sediment either fossiliferous deposits to have accumulated during an by subsidence and progradation or by changes in base interglaciation. level. Webb et al. (1996) also proposed that subsequent Agglutinated foraminifera and other marine micro- to deposition, the Sirius Group deposits were uplifted fossils occur in the tillites at the southern end of the by about 1300 m. bluffs below the horizon containing leaves of Notho- A biostratigraphic age of less than 3.8 Ma has been fagus beardmorensis. Webb et al. (1996) correlated the assigned to the MDF based on the occurrence in the foraminifera assemblage with a similar assemblage Oliver Bluffs glacigenic sediments of reworked from the upper part of the Cloudmaker Formation. The marine diatoms (Harwood, 1986; Webb et al., 1996). Cloudmaker Formation, the lower part of the Sirius The same diatom taxa occur in situ with tephra in the A.C. Ashworth, D.J. Cantrill / Palaeogeography, Palaeoclimatology, Palaeoecology 213 (2004) 65–82 69 CIROS-2 core from the Ross Sea with 40Ar/39Ar ages abundant achenes (all

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